Since tissue expansion was introduced over thirty years ago, the technique has become recognized as an established method of reconstruction with wide applications. Tissue expansion is used to increase the surface area of skin. The need for the technique arises when additional skin is required to repair a defect or when the overlying skin must be stretched to accommodate a large prosthesis such as a mammary implant.
Early efforts at tissue expansion utilized a simple inflatable balloon fitted under the skin in the area to be enlarged (Newmann J. Plastic & Reconstructive Surgery, 1957). It soon became apparent that such a device presented certain problems which included wrinkling of the shell during placement and a tendency for the device to expand laterally during inflation. Current art devices overcame these limitations by incorporating a base into the device which, being substantially less flexible than the thin overlying shell, tends to hold the device flat during insertion and, if reinforced as with DACRON mesh, restricts lateral expansion during inflation (See patents to Perras, Radovan).
Actually, bases have been present in tissue expansion devices ever since the dipping process of manufacturing shells has been used. In the dipping process, a mandrel shaped to the desired contour and volume of the fully expanded device, is dipped into an elastomeric dispersion and precured between successive dips. The dispersion coating the dipped mandrel runs downward toward the mandrel support during the pre-cure thus thickening the shell around the mandrel support. Since the mandrel is dipped several times to build up the shell to the desired thickness the area of the shell surrounding the mandrel support undergoes repeated thickening so that when the shell is peeled off the mandrel and the removal hole patched, the shell is unavoidably thicker in the area of the patch. Such a base, while extensible, is substantially thicker than the thin overlying shell and assists in preventing wrinkling during insertion of the device under the skin. It does little, however, to control undesired lateral expansion.
Radovan (U.S. Pat. No. 4,217,889) describes a tissue expansion device incorporating a substantially nonextensible reinforcing material into the base to control lateral expansion. The Radovan base is variously described as "stiffly flexible" and "substantially non-extensible" and as "being substantially thicker than the thin overlying cover". When a Radovan device is implanted under the skin and inflated, the overlying skin stretches as desired. The inflated shell exerts an upward pull on the margins of the device defined by the periphery of the base material causing the margin to lift upward toward the overlying skin. Because of the difference in flexibility of the cover and base, a ridge forms at the margin. Such marginal uplifting can exert undue pressure against the thin overlying skin with necrosis and subsequent erosion.
Surprisingly, it is found that by incorporating a marginal tensioning ring into the interior chamber of a shell of substantially uniform thickness; that is, as uniform a thickness as is possible using the dipping technique to make the shell, that the resultant device has all the desired operative properties of current art devices, e.g. removal of wrinkles from the shell, ease of placement, control of lateral expansion, and demonstrates minimal tendency toward undesired marginal uplifting.
It is one object of the present invention to provide a tissue expansion device which contains all the desired aforesaid attributes of current art devices and present a further attribute of reducing tissue damage due to marginal uplifting. It is a further object of this invention to provide a relatively inexpensive tissue expansion device.